With developments in laser oscillators, laser processing technology for subjecting materials to removal processing, bonding processing, and surface reforming using a laser beam, is gaining widespread use. In particular, laser cutting technology, which is one of the technologies for processing metal materials using a high power output laser beam, has various advantages as follows: post-treatment is not necessary; automation is possible; and high-speed cutting is possible. Accordingly, the laser cutting is widely used in industries for cutting various types of materials.
Hereinafter, a laser processing apparatus of the prior art will be briefly described.
FIG. 24 is an explanatory diagram illustrating a laser processing apparatus of the prior art, which is used for cutting a steel plate. In a laser processing apparatus 900 of the prior art, a CO2 laser oscillator 901, which uses CO2 gas as a laser medium, is often used as a laser oscillator. A laser beam LB emitted from the CO2 laser oscillator 901 is transmitted through an optical path provided inside a main body 903 and a processing head 905 of the laser processing apparatus 900, and is focused and emitted onto a surface of a steel plate S, which is a workpiece.
Specifically, as shown in FIG. 24, a laser beam LB having a wavelength of 10.6 μm emitted from the CO2 laser oscillator 901 is reflected and transmitted by mirrors 907a to 907d provided on the optical path, and is focused onto the surface of the steel plate by a focusing lens 909 provided inside the processing head 905. The steel plate S which has absorbed the laser beam LB is heated by energy of the laser beam LB and is melted. Assist gas such as oxygen or inert gas is sprayed from a nozzle equipped at the end of the processing head 905 in the coaxial direction with the laser beam LB, and in this way, the molten steel is removed to a lower part (in the negative direction of the Z-axis in FIG. 24) of the workpiece.
The laser processing apparatus 900 translates the focus point of the laser beam LB within the XY-plane while emitting the laser beam LB, and thereby continuously melting and removing the workpiece material. As a result thereof, a part of the workpiece at or near a translation line of the laser beam LB is removed, and the workpiece is finally cut. Here, the part of the workpiece that has been removed by the irradiation with the laser beam LB is referred to as “kerf”. Further, polarization of the laser beam emitted from the CO2 laser oscillator 901 is generally made circular polarization, in order that the processing is kept uniform even in a case where the workpiece is cut in any directions on the XY-plane.
In the translation in the X-axis direction performed by the laser processing apparatus 900 is realized by the main body 903 of the laser processing apparatus 900 moving along a guide rail 911 together with the laser oscillator and the processing head. Further, the translation in the Y-axis direction performed by the laser processing apparatus 900 is realized by moving the processing head 905, the processing head 905 including the mirror 907d, the focusing lens 909, a nozzle that sprays assist gas, and the like.
FIG. 25A is a top view when a state in the vicinity of the focus point of the laser beam LB is seen from the upper part of the steel plate S and is enlarged, and FIG. 25B is a cross-sectional view along the center line of a kerf 921 shown in FIG. 25A.
One laser beam LB focused from the upper part is, as shown in FIG. 25A and FIG. 25B, absorbed into the forefront of the kerf (hereinafter, also referred to as kerf front) 923. The laser beam LB is absorbed into the workpiece, and thereby causing melting of the material. The kerf front 923 moves in a cutting movement direction. Further, as shown in FIG. 25B, the position of the kerf front at the bottom surface of the workpiece is delayed compared to the position of the kerf front at the top surface of the workpiece. Accordingly, at the kerf front, a tilted kerf front surface 925 is formed. As shown in FIG. 25B, the molten material (for example, molten steel) runs down the kerf front surface 925 with a force of the assist gas (not shown) that is supplied coaxially with the laser beam LB, and is removed to a part lower than the bottom surface of the material. With the translation of the focus point of the laser beam LB, this phenomenon continuously occurs, thereby forming the kerf 921.
As schematically shown in FIG. 25A and FIG. 25B, after the laser beam LB has passed and the kerf 921 has been formed, a striation 927 remains, on the side surface of the kerf, in a direction perpendicular to the cutting direction. The structure of the striation has certain irregularities and degree of roughness (roughness). The roughness is an important index in evaluating quality of a cut surface in the laser cutting processing, and it is desired that the value of the roughness be minimized.
On the other hand, recently in the field of high power laser processing, solid-state lasers that each emit a laser beam having a wavelength around 1 nm (about 0.5 nm to 2 nm, hereinafter, referred to as “1 nm band” in this description), such as a fiber laser and a disc laser, have increased the power outputs thereof, and have gained attention. Those lasers can each focus a laser beam smaller than a CO2 laser emitting a laser beam having a wavelength of 10.6 nm which has been used conventionally, and as a result thereof, the heating of the workpiece with a higher energy density can be performed. Therefore, those lasers are suitable for high speed processing. Further, those lasers that each emit a laser beam having a wavelength in the 1 nm band have a characteristic that transmission through an optical fiber is available.
As described with reference to FIG. 24, in the laser processing apparatus including a CO2 laser, since it is necessary that the laser beam is reflected by mirrors to be transmitted, it is necessary to drive together the body of the laser oscillator when translation of the focused laser beam was performed, and hence, a huge machine was required. On the other hand, in the case of the laser emitting a laser beam having a wavelength in the 1 nm band capable of being transmitted through an optical fiber, the laser oscillator can be removed from the processing apparatus, and the laser beam can be transmitted through the optical fiber without using the mirrors. Accordingly, the cost needed for manufacturing the machine part can be reduced by the simplification of the apparatus. In addition, the oscillator of the CO2 laser has an oscillator structure in which a laser medium is disposed between two resonators, and hence requires regular resonator alignment adjustment works. On the other hand, the fiber laser and the disc laser have advantages of not practically requiring the resonator alignment adjustment, and thus of being capable of enhancing the working efficiency of the laser processing apparatus. As described above, the advantages obtained by introducing the fiber laser and the disc laser in the laser processing apparatus are tremendous.
However, regarding the cutting of a thick steel plate having a thickness of about 4 mm or more, there is a problem that the level of cut quality obtained by a laser emitting a laser beam having a wavelength in the 1 μm band is lower than the level of cut quality obtained by the CO2 laser. For example, Non-Patent Document 1 reports comparison results obtained by cutting a stainless steel plate using a disc laser and a CO2 laser, and discloses that with a stainless steel plate having a thickness of 6 mm, the cut surface roughness obtained by using the disc laser is extremely greater compared to the cut surface roughness obtained by using the CO2 laser. From the viewpoint of this issue of the quality of a cut surface, the present state is that the CO2 laser is still being used for cutting a thick steel plate with a laser beam, and the cutting of a thick steel plate is not receiving the above-mentioned advantages of using the laser emitting a laser beam having a wavelength in the 1 μm band.
As a suggestion for reducing the roughness of the surface cut by the laser beam having a wavelength in the 1 μm band, Patent Document 1 points out first that beam quality of a laser beam is generally satisfactory in the CO2 laser, and discloses that the cut quality can be enhanced even with the laser beam having a wavelength in the 1 μm band, if a laser beam has satisfactory beam quality (also referred to as beam parameter product (BPP)). However, the fact is clear as disclosed in Non-Patent Document 1, that even when the fiber laser or the disc laser emitting a laser beam having a satisfactory beam quality is used, the quality of the cut surface does not reach the level of quality obtained when the CO2 laser is used. This suggests that the difference in the wavelengths of the laser beams, that is, 1 μm band and 10 μm, changes the cutting phenomenon largely, and also shows the complexity of the issue that it is difficult to apply the knowledge of the cutting using the CO2 laser of the prior art to the cutting using the laser emitting a laser beam having a wavelength in the 1 μm band.
On the other hand, with the use of multiple laser beams each having a wavelength in the 1 μm band transmitted through an optical fiber, spatial intensity distribution of the laser beams can be controlled, and studies have been made for applying the method of controlling the spatial intensity distribution to the laser cutting.
For example, Patent Document 2 discloses technology for obtaining, by placing multiple laser beams in a ring shape, the intensity distribution in which circular laser beams are arranged in the ring shape. According to this technology, sharp rise in laser intensity distribution is realized at the outer edge part of the laser beams, and the technology has a certain effect on roughness suppression with respect to the cutting of a steel plate using assist gas containing oxygen gas as a main component. However, with respect to the cutting using, as the assist gas, gas containing as a main component inert gas such as nitrogen gas or argon gas, which is mainly used for stainless steel or the like, it is difficult to provide a roughness similar to the roughness obtained by using CO2 laser and to exhibit sufficient effects.
Further, Patent Document 3 suggests technology of placing at least one beam for molten material ejection at a rear part of a main laser beam that mainly generates a molten material to assist ejection of the molten material. As disclosed in Patent Document 3, a basic way of thinking this technology is as follows. A surface of the molten material is vaporized by the irradiation with the beam for molten material ejection, and the ejection of the molten material is made to be performed smoothly using high pressure obtained as a reaction force of the vaporization. By making the ejection of the molten material to be performed smoothly, cutting capacity can be enhanced, including increases of cutting speed and cut thickness. However, regarding the quality of a cut surface, the quality chiefly depended on the main laser beam used for generating a molten material, except for a deep part near to the bottom surface of the material at which an action of the beam for molten material ejection appears. Thus, no significant quality enhancement could be achieved compared with the case of using a usual one beam. Further, according to this technology, the spatial intensity distribution of laser beams focused on a surface of a material was asymmetric. Accordingly, in order to support uniform cutting characteristics in any directions, an accurate mechanism was necessitated for rotating the spatial intensity distribution of laser beams in accordance to the change of the cutting direction, and hence, there was a drawback that the apparatus increased the complexity thereof.